1. Field of the Invention
This invention relates to electromagnetic ballasts for gaseous discharge lamps and particularly to such ballasts for starting and operating serially connected fluorescent lamps.
2. Description of Related Art
The voltages which a ballast must provide to start and operate serially connected gaseous discharge lamps, e.g. fluorescent lamps, are substantially larger than those needed for single or parallel connected lamps. These voltages become even larger for higher wattage lamps and/or for lamps which must start and operate at low ambient temperatures. As a consequence, the lamp voltages might exceed the voltage ratings of fixtures and/or hardware for holding the lamps.
It is an object of the invention to provide an electromagnetic ballast which is capable of starting and operating serially connected gaseous discharge lamps at open circuit lamp voltages which exceed the rated voltages of lamp holder apparatus, but without imposing such high voltages on the lamp holder apparatus itself.
In accordance with the invention an electromagnetic ballast for starting and operating a plurality of gaseous discharge lamps, which are serially connected between a first lamp holder connection and a second lamp holder connection, comprises a transformer including a primary winding for electrical connection to an AC power source and a secondary winding. The primary winding includes first and second subwindings. The first subwinding has a first end for electrical connection to the first lamp holder connection and to the AC power source and has a second end. The second subwinding has a first end electrically connected to the second end of the first subwinding and has a second end for electrical connection to the AC power source. The secondary winding has a first end electrically connected to the second end of the first subwinding and to the first end of the second subwinding and has a second end electrically connected to the second lamp holder connection. The windings are wound such that, in operation, voltages produced by the first subwinding and the secondary winding are additive, but voltages produced by the second subwinding and the secondary winding are subtractive, whereby the voltage at each of the lamp connections is substantially lower than the voltage across the serially connected lamps.